Cytokines and chemokines are important elements of functional immune systems. For example, interferon is one of biotechnology's first recombinant drugs and is used as an antiviral agent and immunoadjuvant for vaccines in animal health (2;24). Gamma interferon (IFN-γ) is a cytokine that elicits potent antiviral and immunoadjuvant responses in animals (1-3;6;8;13;17;18;20;22;23;25). IFN-γ-enhanced vaccines (13;18) are useful in the treatment of diseases such as, for example, shipping fever and mastitis in cattle. However, under current technology IFN-γ has proven to be both unstable and extremely expensive to produce. With costs in the hundreds of dollars per milligram, and required treatment levels in multiple milligrams per dose, the use of IFN-γ as an animal health antiviral agent or as a vaccine adjuvant is deemed impractical. The initial promise of the interferons and other cytokines (set forth in Table I), as magic bullets for curing disease, has yet to be fully realized (1).
In cattle and other animals, such as other mammals, birds, fish, and reptiles, IFN-γ acts either directly or indirectly on almost every component of the innate and adaptive immune systems (1). In addition, IFN-γ is one of the most, if not the most, pleotropic of the cytokines, profoundly affecting antigen processing and presentation, inhibition of lymphocyte migration, macrophage activation, B-lymphocyte antibody production (21), natural killer (NK) cell activity, and upregulation of leukocyte cell-surface molecules for trafficking and immune recognition. Strong receptors for IFN-γ are located on T and B-lymphocytes, NK-cells, monocytes, macrophage, fibroblasts, neutrophils, endothelial cells, and smooth muscle cells. Also, because of its central role as an antiviral agent, IFN-γ is a major target for viral subversive activity. For example, viruses encode proteins that can inactivate IFN-γ, interfere with IFN-induced antiviral pathways, and interrupt intracellular IFN-γ signaling.
Biologically active bovine IFN-γ was first cloned and synthesized in Escherichia coli in 1986 (5). The nucleotide sequence of equine IFN-γ was reported in 1994, showing a sequence identity of 67% to human and 78% to bovine IFN-γ. The structure of recombinant chicken IFN-γ was reported in 1999, and an active, truncated form (truncated at lys 133) was expressed in E. coli. The 3-D structure was shown to be similar to bovine and human IFN-γ, despite an overall amino acid identity of only 32% (14).